Page:EB1911 - Volume 21.djvu/751

Rh in width. The word comes through the Fr. planche (from post-Augustan, Lat. planca, a nasalized adaptation probably of Gr., something flat, especially a flat stone. The use of the word “plank” in the sense of an article in a political programme is of American origin and is due to the use of “platform” for the programme itself.  PLANKTON, a name invented by Professor Victor Hensen for the drifting population of the sea. This is a convenient heading under which to discuss not only plankton proper, but the benthos, or crawling population of the sea-bottom. Scientific investigation of these subjects dates from the reports of the “Challenger” expedition, which, despite its many successors, still stands out as the most important of the oceanographic expeditions, alike by the work achieved, the distance traversed, the time occupied, and the money devoted to the publication of the results. It laid the foundation of our knowledge of the physics and chemistry of ocean water, of oceanic and atmospheric currents, of the contour of the sea-bottom, and of the main features of distribution of deep-sea life. Later work has confirmed and expanded, but not revoked, the conclusions thus attained. But, in spite of this and of several subsequent expeditions, it cannot be pretended that we are in a position to formulate general canons of marine distribution other than of the most tentative character. Two fallacies underlie many attempts to define distributional oceanic areas for special groups: the one, that such areas can be made to bear some relation to existing geographical or even national divisions; the other, that what is true for one group of the animal kingdom must hold good equally for another. It is necessary at the outset to divest oneself of these errors; oceanic conditions depend only very indirectly upon the distribution of the land, and strongly swimming or freely floating animals are not to be confined by the same factors as determine the distribution of sessile forms, whose range is governed by a variety of circumstances.

As Wyville Thomson pointed out long ago, there is but one ocean. This surrounds the southern half of the globe, and has two large gulfs, generally called the Atlantic and Pacific Oceans, which meet through narrow channels in the small Arctic Ocean, and a half gulf, the Indian Ocean. The Atlantic and Pacific exhibit a striking homology of atmospheric pressure and of prevalent wind and current; the Indian, to a great extent, resembles the southern half of a larger one, but this resemblance is modified by the neighbourhood of vast land masses. The prevalent winds, dependent on the fairly constant distribution of atmospheric pressure over the great oceans, are the most important determinant of currents. As at most points in the ocean the temperature, salinity and chemical composition of the water are mainly determined by the currents—that is, by the condition at the place whence the water came—it is obvious that a study of currents must precede any general view of the distribution of marine forms.

Regard must be had not merely to the superficial currents indicated in fig. 1, but also to the movements of the deeper layer. Ice melting at the poles, together with polar precipitation of hail, snow and rain, yields large quantities of water of low salinity and very low temperature; this water sinks under the warmer salter surface water drifted from lower latitudes, and, creeping slowly north and south from the poles, covers the bottom of all the great open oceans at very uniformly low temperatures (in some cases as low as 30° F.). Between surface and bottom the temperature gradually decreases (except where affected by local circumstances), and in the middle layers the existence of slow currents is suspected. The cold bottom water wells up to the surface in certain areas, replacing the surface water drained away by currents, notably to the westward of the great land masses. Ocean water is remarkably uniform as regards its contained salts and gases, and it does not seem likely that we can look to these to explain the facts of distribution. In its temperature, on the contrary, there is enormous variation. While the bottom water of the ocean is very cold, and the midwater of a more or less intermediate temperature, the surface water, according as it has drifted from the equator polewards

or in the reverse direction, has a mean annual temperature somewhere between 84° and 30° F., losing or gaining heat on its way. In the case of narrow or “closed” seas, and near land masses, sea-water does not exhibit that uniformity of composition which characterizes the open ocean; but even in such cases the temperature is largely influenced by adjacent currents, and, though less obviously than in the open ocean, seems to be a very important agent in distribution.



The fauna of the sea is divisible into the plankton, the swimming or drifting fauna which never rests on the bottom (generally taken now to include E. Haeckel's nekton, the strong swimmers, such as fish and cephalopods), and the benthos, which is fixed to or crawls upon the bottom. These groups require a further subdivision according to depth—the more necessarily since, to some zoologists, any water over 100 fathoms is “deep” or even “abyssal.” It is simplest to begin with the benthos. From the shore seawards we may distinguish several zones. Even the tidal zone, between high and low water-mark, is sub divisible by its fauna and flora. There generally follows on this a very gentle slope to the depth of about 100 fathoms, locally subdivisible into many lesser zones. It has been termed the continental shelf or littoral zone, not very appropriately, since it occurs round many oceanic islands, and even away from any land. In this zone, if near land, fall to the bottom the heavy materials produced by land Waste and river drainage. The fauna of this zone, generally very well characterized, may be